1. Field of the Invention
The present invention relates to a linear light source of an image sensor that is used in an image input section of a facsimile apparatus etc. The present invention also relates to a contact image sensor having such a linear light source.
2. Background Art
FIGS. 9-11 show a conventional image sensor that is listed in the Mitsubishi contact image sensor catalogue (produced in February 1993), for example. FIG. 9 is a sectional view of the contact image sensor, FIG. 10 shows a linear light source of the contact image sensor, and FIG. 11 shows the directivity characteristic of each LED chip that is used in the linear light source of the contact mage sensor.
In FIG. 9, reference numeral 10 denotes an original document; 20, a linear light source in which LED chips are arranged linearly; 30, an erect 1:1 imaging rod lens array, or an erect-image, equimagnification imaging rod lens array, that is composed of a plurality of rod lenses (not shown); 40, a sensor board; 50, sensor ICs attached to the sensor board 40 so as to be arranged linearly; 60, a glass plate on a document running surface; 70, a sensor frame; and 80, a path of light that is output from the linear light source 20 and imaged on the sensor ICs 50.
In FIG. 10, reference numeral 90 denotes a LED bare chip; 100, substrate that is mounted with the respective LED bare chip 90; and 110, a resin body with which the respective LED bare chip 90 is sealed. The surface of each resin body 110, from which light generated by the corresponding LED bare chip 90 is emitted, is flat. An LED bare chip 90, a substrate 100, and a resin body 110 constitute a surface mounting type LED chip 120. Reference numeral 130 denotes a circuit board on which a plurality of LED chips 120 are mounted linearly. The plurality of LED chips 120 and the circuit board 130 constitute an LED array linear light source 20. Reference numeral 140 denotes an exposed surface (document surface) that is illuminated by the LED chips 120; 150, the distance between adjacent LED chips 120; 160, paths of light that spreads radially from each LED chip 120; 170, the distance between each LED bare chip 90 and the document surface 140; 180, an exposure profile of one chip that is obtained when the illumination surface 140 is illuminated by light coming from each LED chip 120; and 190, a total exposure profile as the sum of exposure profile 180 of each chip.
Next, the operation will be described with reference to FIGS. 9-11. Light that is output from the linear light source 20 passes through the glass plate 60 and illuminates the original document 10 uniformly. The exposed light is reflected by the original document 10 in accordance with light-and-shade information of an image and goes along the path 80 of light. The reflected light passes through the rod lenses of the rod lens array 30 and is imaged on the sensor ICs 50. The sensor ICs 50 accumulate charges in accordance with the intensity of the reflected light and outputs signals via the sensor board 40. The linear light source 20 is realized by arranging the point light sources of LED chips 120, and is composed of the LED bare chips 90, the substrates 100, the resin bodies 110, and the circuit board 130. Each LED chip 120 has a wide directivity characteristic as shown in FIG. 11, and the exposure profile 180 on the document surface is somewhat convex. The total exposure profile 190 as superimposition of the exposure profiles 180 assumes a flat line.
The conventional linear light source is configured in the above-described manner, and each LED chip 120 has the wide directivity characteristic as shown in FIG. 11. Although the directivity characteristic of FIG. 11 is the one taken along the surface of the drawing of FIG. 10, each LED chip 120 exhibits approximately the same directivity characteristic in the direction perpendicular to the surface of the drawing of FIG. 10. As for the exposure in the direction perpendicular to the surface of the drawing, it is more efficient to illuminate a reading position in a concentrated manner. However, conventionally, priority is given to uniform illumination of a document and the efficiency of light utilization is sacrificed in the direction perpendicular to the surface of the drawing of FIG. 10.
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to provide a linear light source that has a uniform exposure characteristic and is superior in efficiency of light utilization.
According to one aspect of the present invention, a linear light source comprises a circuit board and a plurality of LED chips arranged linearly on the circuit board, and each of the LED chips has a light condensing function.
In another aspect, in the linear light source, an arrangement interval between the adjacent LED chips is preferably shorter than or equal to a distance between the LED chips and a document exposure surface.
In another aspect, in the linear light source, a ratio between the interval of the adjacent LED chips and the distance from the LED chips to the document exposure surface is preferably determined according to a directivity characteristic of each of the LED chips and an equation
L(xcex8)=L(0xc2x0)/(2xc2x7cos2xcex8)
where L(xcex8) is exposure at an angle xcex8, and xcex8 is an angle with respect to a perpendicular to the document exposure surface from the LED chips.
Other and further objects, features and advantages of the invention will appear more fully from the following description.